In this article we will discuss about Trochophore Larva:- 1. Historical Retrospect of Trochophore Larva 2. Structures of the Trochophore Larva 3. Biology and Metamorphosis 4. Structures 5. Affinities 6. Phylogenetic Significance.
Historical Retrospect of Trochophore Larva:
1. Loven (1840), a Swedish naturalist, was the first man who discovered trochophore larva. Since then the larva was known as Loven’s larva.
2. Semper (1859) used the name Trochosphaera, a rotifer for the organism.
3. Ray Lankester (1877) gave the name ‘Trochophore’ to this larval form.
4. Afterwards it was Hatschek (1879) who also supported the most accepted name ‘Trochophore’.
5. Hyman (1957) and Barnes (1980) tried to establish relationship of trochophore with other groups of animals.
Structures of the Trochophore Larva (Loven’s Larva of Polygordius):
All the archiannelids (Polygordius and other species) are known to pass through the larval stage—the trochophore (Fig. 17.12A-C).
1. Marine planktonic and most are lecithotrophic larvae.
2. The anterior end of the body is broader than the posterior end and it exhibits bilateral symmetry.
3. It has mouth, alimentary canal and anus.
4. The mouth is situated near the mid-ventral line of the body and leads into the sac-like stomach and proceeds as a narrow alimentary canal. Both the walls of the alimentary canal extending from mouth to anus are lined by cilia.
5. The canal ends in an anal aperture at the posterior tip of the body.
6. Two prominent bands of cilia encircle the body and in certain forms a third band may be present. These ciliary bands are used in locomotion and feeding.
7. A circlet of strong locomotor ciliary band, called the pre-oral circlet or Prototroch (Fig. 17.12A), encircle the body around the middle and lies above the mouth, and the metatroch which is a ciliary transverse ring behind the mouth.
8. The prototroch arises from special cells, called trochoblasts.
9. There is often a second circlet of cilia around the pygidium or anus, called the telotroch.
10. A longitudinal band along the mid- ventral part of the body is also present in some cases and is called a neurotroch.
11. Trochophore exhibits no metamerism and the rudiment of future adult trunk is seen as a small region at the posterior pole.
12. There is no coelom at this stage but only a spacious blastocoel encloses the gut.
13. Within blastocoel, a pair of protonephridia, certain amount of mesenchyme and larval muscles is present.
14. The nephridia are made of two hollow cells; each contains a flame of cilia. One nephridium lies on each side of the gut.
15. The upper pole possesses an ectodermal, thickened area, called apical sensory plate which contains cells that are the primordia of the cerebral ganglia.
16. At the apical end, a number of long cilia emerge from the apical plate, called apical tuft of cilia.
17. Many trochophores bear sense organs, such as ocelli (eye spots) below the apical plate region.
18. The fully developed trochophore larva may be divided into three regions, such as the pretrochal region which includes the area above the prototroch. The posterior region of the larva is called pygidium which includes telotroch and anal area. The middle region is called growth zone which includes the area between the mouth and telotroch.
Biology and Metamorphosis of Trochophore Larva:
The free-swimming trochophore larva of some forms (e.g., Echiurans, some polychaetes, Polygordius, phyllodocids, serpulid fan worms) feeds on the plankton and other microscopic marine organisms and the trochophore is called planktotrophic larva and has long free-swimming life.
In some other groups (e.g., Sipunculans, nereids and eunicids of polychaetes), the trochophore larvae are lecithotrophic larvae. They do not take any food from external sources and mainly feed upon the yolk originally laid down in the egg. The lecithotrophic larvae lead a short planktonic life. Metamorphosis of the larvae is best seen in Polygordius.
The first sign of metamorphosis is marked by the segmentation of mesodermal bands. Later the posterior region elongates rapidly and is externally marked with segmentation. The area above the prototrochal ring becomes prostomium and the prototrochal area differentiates as peristomium.
The apical sense organ area becomes the cerebral ganglion which is joined with the ventral nerve cord. Internally the mesodermal band splits to produce coelomic sacs.
The mouth shifts forward and the anal organization changes gradually. The ciliary bands disappear and the larva grows in size and length with addition of the new segments. After metamorphosis the young worm sinks to the bottom of the sea and becomes the burrowing adult worm.
Structures of the Trochophore Larva in Different Classes:
1. The larva of Neanthes (= Nereis) is similar to the typical trochophore but with a pair of eye spots.
2. Larvae with no blastocoel but the ectoderm and endoderm are in contact except where they are separated by the larval mesoderm, e.g., larva of Psygmobranchus.
3. Cilia are either rarely distributed over the whole surface of the body or are not confined to special circlets, called atrochal larva, e.g., larva of Lumbriconereis.
4. In the larva of Nephthys, two circlets of cilia are seen, one at each end of the body, the pre-oral (anterior) and perianal (posterior), called telotroch larva.
5. The complete rings when may be present on both dorsal and ventral surfaces, called amphitrochal larva.
6. In the larva of Chaetopterus where one or more rows of cilia surrounds the middle of the body, called mesotrochal larva. In mesotrochs the pre-oral and peri-anal rings are absent.
7. In the larva of Ophryotrocha, there are many ciliary circlets and each develops on a true mesodermal segment hence called polytrochal larva.
8. In the larva of Mitraria, long provisional setae are found which are replaced by permanent structures. The older larva of Nereis possesses parapodial-like lateral flattened structures with setae.
No free larval stage is noticed.
There is no larval stage and the development is direct.
Affinities of Trochophore Larva:
This larval form exhibits remarkable similarities with several other larval forms. As a consequence the phylogenetic status of Trochophore warrants serious consideration.
Affinities of Ctenophora:
The aboral sense organ (Statocyst) of a ctenophore is compared with the apical sensory plate of trophophore. The sub-ectoder- mal radiating nerves are comparable. The prototroch is derived from fourth group of ciliated cells. Both of them have pear-shaped body.
Despite the similarities the fundamental organisation portrays many diversities. The cleavage pattern is different in both the cases. The anus is absent in ctenophores. So the trophophore larva cannot be regarded as related to ctenophores.
Affinities with Muller’s larva:
The Muller’s larva of Turbellarians especially that of Planocera, shows similarities with the trochophore larva. Similarity in developmental stage, similarity in the disposition of ciliated bands and presence of eye spots at the aboral end of the two larval forms led many workers on this line to draw parallelism between the two groups. But due to undernoted dissimilarities the parallelism cannot be justified.
The dissimilarities are:
(i) Absence of anus in Muller’s larva,
(ii) The enter on opening into one opening in Muller’s larva,
(iii) Difference in the embryonic differentiation of mesoderm and
(iv) The existence of tuft of cilia at the caudal end of Muller’s larva.
Affinities with Pilidium (Nemertini) larva:
The pilidium larva of Nemertini exhibits certain similarities with the trochophore larva.
The similarities are:
(i) Both have helmet-shaped body,
(ii) The ciliated ring between aboral and oral ends of pilidium larva represents the prototroch of trochophore,
(iii) Similarities in the disposition and distribution of nerve ring,
(iv) The stomodaeum shows similarities,
(v) The schizocoelic mode of formation of coelom in both.
But the absence of anus in pilidium and the dissimilarities in the formation of mesoderm stand on the way to draw any relationship between them.
Affinities with Rotifera:
Trochosphaera, a rotifer, shows some similarities with the trochophore larva of annelid. Trochosphaera resembles trochophore in many respects, viz., ciliated girdles, disposition of nervous system (‘Brain’) and the sense organ, placement of anus, nephridia and curvature of intestine. But the resemblances are mostly superficial in nature and need critical examination to draw any phylogenetic relationship.
Affinities with Veliger larva:
The pre-oral ciliated ring, ciliated tuft of flagella and apical plate of the veliger larva of mollusca are similar with that of trochophore larva. The similarities between the trochophore and veliger larva are possibly due to remote phylogenetic convergence.
Phylogenetic Significance of Trochophore Larva:
In the evolutionary dynamics of invertebrates the trophophore larva occupies a prominent status. It shows similarities with many invertebrate groups. The affinities throw light on the emergence of bilateral groups from the animals having radial symmetry (Fig. 17.13).
It is claimed that the trochophore represents a transitional stage in the line of emergence of the bilateral groups (e.g., Rotifers) from the radial groups (Ctenophores). Similarities between the trochophore and the echinoderm larva (Bipinnaria and Pluteus) and Tornaria larva of Balanoglossus added more weight to this contention.
Many workers are of the opinion that the Trochophore larva serves as a bridge between radial and bilateral symmetry. They have opined that the bilateral symmetry has evolved from the radial one.
Regarding their views, there are many theories which may be given below:
1. Ctenophore—Polyclad theory (proposed by Lang, 1881)
2. Ctenophore—Trochophore theory, later modified by Hatschek (1878)
3. Planuloid—Coeloid theory (proposed by L. Vongraff, 1882).
Second theory is more or less acceptable by the workers. According to this theory Trochophore larva arose from the hypothetical animal—Trochozoon. Again, Salvini—Plawen L. (1973) has stated that annelids and echiurans are closely related by their larval stage (trochophore larva) whereas flatworms, nemerteans and entoproct larvae are unrelated.